1. Field of the Invention
The present invention :elates generally to a method and apparatus for manufacturing compression molded articles from thermoplastic material. More specifically, the present invention relates to such a method wherein the thermoplastic material is heated, a blank is molded by means of a die which may be annular, and the blank is separated from the die using a heated stream of gas which surrounds the die and is directed at the point of separation of the blank from the die. The blank is introduced in a compression molding mold which is then closed, the compressed article is molded by application of a molding pressure, and cooled at least partly under pressure. The apparatus of the present invention relates to an apparatus for carrying out the inventive method having an extruder for heating thermoplastic material with a closable die suitable for dispensing portions of the molding material which may be annular, and which may be surrounded by an annular nozzle for discharging a heated stream of gas directed at the point of separation of the material portions from the die, at least one closable compression molding mold consisting of a molding matrix and a molding mandrel, wherein the molding matrix can be arranged beneath the die in one process step and coaxially with the molding mandrel in another process step, whereby a molding pressure is applied to and maintained on the molding mandrel and/or the molding matrix.
2. The Prior Art
A method such as discribed above is disclosed in U.S. Pat. No. 3,313,875, to Magerle, granted Apr. 11, 1967. In the method according to this patent, the thermoplastic material is dispensed by an annular die and applied to a projection of the die, so that the connection with the gap of the die is maintained and the molding material adheres to the projection of the die, from where it is later stripped when the mold is closed. Thus, the blank is cooled even before it is passed into the mold, on the one hand, and problems are encountered, on the other hand, with the stripping of the molding material from the die because of the adhesion and sticking of the thermoplastic material to the projection of the die, such problems interfering with the function of the stripper.
A similar method is described in German patent No. DE 30 23 415, in which the blanks, which are separated from the die by the closing of the latter, are admitted into the hollow space of the molding press by being permitted to free fall into the space. In this way, separated blanks do not come into contact with the cooled surfaces of the mold as such portions drop down. However, the blanks do dwell in the cooled mold for a period of time amounting to several times the dropping time, i.e., until the mold is removed from the die and brought into axial position with the molding plunger. Thus, since the blanks have relatively large areas of contact with the cooled mold, the plastic material starts to cool in the areas of contact, which adversely affects the compressibility of the plastic material and the quality of the manufactured molded articles. If the die and/or its closing elements are not finished with the highest degree of precision, or if they are not clean and have foreign matter sticking to them, the blank cannot be separated simultaneously along the entire circumference thereof, so that it does not drop into the hollow space of the mold with precise alignment, which by itself or in combination with parts of the plastic compound that have prematurely cooled on the walls of the mold, results in unevenly molded articles which must be rejected. The same unacceptable result can be achieved, if the annular stream of gas, if any, directed at the point at which the blank is separated from the die, acts unevenly on the blank. The effect of this is first an uneven detachment of the blank, and, second, an acceleration of the blank in a non-axial direction, so that the blank cannot pass into the hollow space of the mold centrally aligned.
Apparatus is known from U.S. Pat. No. 3,313,875, DE 30 23 415, and also from U.S. Pat. No. DE 28 42 515, for carrying out the method which is afflicted with the above-mentioned problems.